The present invention relates generally to reading/writing data on a data storage medium. More particularly, the present invention pertains to an optical time-based servo system and method for positioning a transducing head relative to one or more tracks located on a storage medium.
Electronic data storage devices are known in the art. In general, these devices are classified by how they read and write data. Magnetic data storage devices use a transducing or xe2x80x9cread/writexe2x80x9d head that alters the magnetic state of a portion of a magnetic disk or tape. Optical devices, on the other hand, utilize a read/write head having a low-power laser beam to read and write digitized data encoded in the form of tiny changes in reflectivity on the surface of an optical disk.
At higher track densities, the recording and reading of data in tracks on data storage media requires precise positioning of the read/write heads. The read/write heads must be quickly moved to and centered over particular tracks as reading/writing of data is performed. The heads can read/write data from a particular track as the media moves relative to the transducing head in a transducing direction. To access other tracks, the read/write heads move laterally across the media in a translating direction that is generally perpendicular to the track.
In the case of storage devices that read/write data on magnetic and optical media, servo control systems are sometimes used to accurately position the data read/write heads in the translating direction (e.g., across the width of the tape or across the radius of the disk). The servo control system generally provides a position signal from one or more servo read heads that read position control information recorded in one or more servo tracks on the storage medium. The position control information is then used to accurately center the data read/write heads over the data tracks.
One type of servo system relies on signal amplitude to determine position. An example of such an amplitude-based servo system often found in tape drives is referred to as a boundary system. In boundary systems, at least one servo track or servo track pattern extends along the length of the media (e.g., along the tape or along a circumferential segment of the disk). The servo track is laterally divided into two or more regions separated by distinct boundaries. The distinct regions have different properties that can be detected by the servo read head. For example, the regions may be recorded at different frequencies or phases, or they may contain bursts occurring at distinct times. Generally, the servo head elements straddle the boundary between the regions, and the ratio of the optical or magnetic amplitude of the response of the servo head to each region provides the position signal upon which the track-following servo operates.
While effective, boundary type servos are particularly susceptible to errors in position signal. In addition, servo head instabilities, head wear, localized debris on the head or media, and media defects all contribute to temporary or long-term shifts in the spatial response of the servo head to the recorded servo pattern in the servo track. When this occurs, the servo system cannot effectively track.
Another amplitude-based optical servo system is found on removable magnetic disk drives where the disks are provided with an optically readable servo pattern. Here, an optical servo signal is generated by a laser which is attached to a servo head. The laser forms a pattern of offset spots on the disk which are then reflected to various photodetectors. The intensity of reflected light from the spots is altered when the spots are incident on servo indicator marks on the disk surface. As the disk rotates and the marks pass through the spots, the reflected signal oscillates with an amplitude determined by the relative track position (e.g., maximum when the spots are precisely aligned with the marks and minimum when the spots are exactly between the marks).
While more than adequate, amplitude-based servo systems like those described above are necessarily sensitive to fluctuations in the amplitude of the servo position signal. Accordingly, as the servo read head""s sensitivity to amplitude is diminished over time, its ability to effectively track is hindered.
To address the problems associated with amplitude based servo systems, time-based systems for magnetic storage media have been described. U.S. Pat. No. 5,689,384 to Albrecht, et al., entitled xe2x80x9cTiming Based Servo System For Magnetic Tape Systems,xe2x80x9d issued Nov. 18, 1997, describes a track following servo control system for use with magnetic media tape systems which derives head position information from one or more specially patterned magnetic servo tracks. The servo patterns are comprised of magnetic transitions recorded at more than one azimuthal orientations in a servo track, such that the timing of the servo position signal pulses derived from reading the servo pattern are decoded to provide a position signal used by the servo system to position data heads over desired data tracks of the storage media.
While such time-based servo systems have numerous advantages over boundary and other amplitude-based servo systems, problems remain. For example, depending upon the type of time-based servo pattern recorded in the servo track and the method of recording such a time-based servo pattern, the system may be sensitive to media speed variations at the time the servo pattern is recorded in the servo track. Thus, specialized servo writing equipment having special speed control features is generally necessary to perform such time-based servo recording. Furthermore, the servo writing equipment must be able to write complex servo patterns to the media.
For these and other reasons, conventional time-based servo systems have proven less than optimal for many applications. What is needed is a time-based servo system and method that overcomes the problems discussed above when used with either optical or magnetic media in either disk or tape formats.
To overcome these problems, a time-based optical servo systems and methods according to the present invention are provided. In one embodiment of the invention, an optical detection apparatus for use with a data storage device is provided. The apparatus includes a servo read assembly having two or more elongate optical detection devices spaced apart along a transducing direction in a non-parallel relationship.
In yet another embodiment of the invention, a method for positioning a transducing head adjacent a data storage track on a data storage medium is provided. The method includes moving a storage medium in a transducing direction relative to the transducing head where the storage medium has at least one optical indicator mark thereon. The method further includes positioning the transducing head over the storage medium in a translating direction generally perpendicular to the transducing direction, wherein the transducing head is operatively coupled to an optical servo read head assembly having a first elongate detection device and a second elongate detection device arranged in a spaced-apart, non-parallel configuration. The method further includes determining the position of the transducing head in the translating direction by measuring a time period required for the at least one optical indicator mark to pass between the first detection device and the second detection device. The position of the transducing head in the translating direction is adjusted based on the time period.
In still yet another embodiment of the invention, an optical servo system for positioning a transducing head assembly adjacent a surface of a data storage medium is provided where the storage medium moves in a transducing direction relative to the transducing head assembly. The system includes an optical detection apparatus for determining the position of the transducing head assembly relative to the storage medium in a translating direction which is transverse to the transducing direction. The optical detection apparatus has a first elongate detection device and a second elongate detection device where the first and second detection devices are arranged in a substantially fixed, non-parallel relationship. The first and second detection devices are adapted to detect the passage of at least one optical indicator mark located on the storage medium.
A servo control system for positioning a transducing head assembly adjacent a surface of a data storage medium is also provided. The system, in one embodiment, includes a servo read head assembly for reading at least one servo indicator mark recorded on a servo track of the storage medium and generating a read head signal representative of the position of the servo read head assembly relative to the at least one servo indicator mark. The servo read head assembly includes a first elongate detection device and a second elongate detection device where the first and second detection devices are arranged in a fixed but non-parallel geometric relationship. The system also has a servo decode circuit to receive the read head signal and generate a position signal based thereon, the position signal indicating the position of the servo read head assembly relative to the at least one servo indicator mark. The servo decode circuit receives the read head signal generated from the servo read head assembly as the servo read head assembly is moved in a transducing direction along the storage medium.
In still yet another embodiment of the present invention, a magnetic data storage device is provided. The storage device includes a housing for receiving and mobilizing a magnetic data storage medium therein and a servo control system for positioning a magnetic read or write head adjacent a surface of the storage medium. The servo control system includes a servo read head assembly for reading at least one servo indicator mark recorded on a servo track of the storage medium and generating a read head signal representative of the position of the servo read head assembly relative to the at least one servo indicator mark. The servo read head assembly also includes a first elongate detection device and a second elongate detection device where the first and second detection devices are arranged in a fixed but non-parallel geometric relationship. The system also includes a servo decode circuit to receive the read head signal and generate a position signal based thereon. The position signal indicates the position of the servo read head assembly relative to the at least one servo indicator mark. The servo decode circuit receives the read head signal generated from the servo read head as the servo read head is moved in a transducing direction relative to the storage disk.
Advantageously, the optical servo system of the present invention provides numerous benefits over other time-based and amplitude-based systems. For instance, the time-based system of the present invention is considerably less sensitive to fluctuations in contrast of the media surface. Furthermore, the optical indicator pattern on the media is less complex to produce. In addition, the signal processing is less complex than that required for amplitude-based systems.